Many people purchase with aperture in mind. Therefore, if you have $500 to spend on a scope, then people will tell you to put all that money into aperture; get the biggest scope you can afford. Unfortunately, it is not as simple as this. First of all, aperture doesn't always win, especially at the cost of portability and optical quality. Secondly, more aperture doesn't give you better pictures should you decide on a scope that can do a little astrophotography as well. Lastly, consideration must be given to the mount. A good scope with a bad mount becomes a bad scope.
I believe there are 4 performance areas that you need to consider when deciding on your purchase. The first is for observing deep space objects (DSOs). This is generally where aperture does rule. If this is your primary consideration, then following this advice is not a bad idea. The second is for planetary observing. This puts more of a premium on optical quality and the way you will mount the scope. The third is for astrophotography . Scopes and mounts purchased for this purpose will generally be very expensive; however, some astrophotography can be done with just about any scope. The final performance area simply concerns the issue of portability . The type of objects to be observed is less important than having a scope with premium optics that is carried easily, sets up quickly, and cools down almost immediately.
Depending on the way you intend to use the scope and the budget you have, I've given you a general guide to follow which should be considered, in no way, comprehensive:
It should be noted that any scope purchased for over $ 5000 or so will normally be bought for one specific purpose. For example, you will purchase it either for visual observations or astrophotography, not both. Scopes will be singular in purpose and in use. If you budget for less than this, then you can normally choose a scope that will do everything pretty well. SCTs and apochromatic refractors are versatile scopes, as are EQ mounted Newtonian reflectors.
Therefore, the more money that is spent, the more that you will want to specialize in the scope design. For this reason, many experienced observers will own a wide variety of expensive scopes. They will then use each scope for a specific purpose.
As a beginner who is spending less money, the above chart should help you narrow down your purchase. If you just want to observe DSOs then purchasing a big dob is the best advice. If you want to experiment with astrophotography as well, then you'll want to consider a scope that is more versatile. If you want a quick scope for easy setups, then you'll settle for smaller refractors and dobsonians.
Give mind to the mount that you'll use. Most of the less expensive scopes will come with a mount. But expensive scopes are normally sold optical tube assembly (OTA) only. You'll need to purchase the mount separately. The above chart prices assume that you'll purchase the appropriate mount as well. For example, the 4" astrograph above is placed in the $5000 to $10000 range because it assumes that you'll also purchase the mount necessary for the scope's use. That 4" OTA will generally run you around $3500, but you'll spend at least $2000 on a mount that is stable enough for astrophotography.
Likewise, I assume that any scope purchased for planetary use will also give consideration to an appropriate mount. I recommend an EQ mount or tracking platform for planetary use. That must be factored into your decision.
Remember that this will not be the only scope you'll ever own. There is nothing wrong with purchasing a scope to do only ONE thing. Just make sure you understand that you will probably need to increase your collection of scopes should you decide to branch out. Between the $1000 and $2000 range, you would be wise to purchase a scope with great versatility such as an SCT. It's a good way to get your feet wet in a variety of exploits since it does everything pretty well (not great). But under that price (and over that price) you'll want to specialize a bit. For example, at $300 you'll not find decent scope specifically for astrophotography, but you can certainly get a good DSO scope; in this case, either a medium sized Dobsonian or a smaller rich-field refractor.
As always, whatever your budget is, be sure to include things like finders, eyepieces, and star charts.
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